Pattern selection for full-chip source and mask optimization

ABSTRACT

The present invention relates to lithographic apparatuses and processes, and more particularly to tools for optimizing illumination sources and masks for use in lithographic apparatuses and processes. According to certain aspects, the present invention enables full chip pattern coverage while lowering the computation cost by intelligently selecting a small set of critical design patterns from the full set of clips to be used in source and mask optimization. Optimization is performed only on these selected patterns to obtain an optimized source. The optimized source is then used to optimize the mask (e.g. using OPC and manufacturability verification) for the full chip, and the process window performance results are compared. If the results are comparable to conventional full-chip SMO, the process ends, otherwise various methods are provided for iteratively converging on the successful result.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority from U.S. Provisional ApplicationNo. 61/255,738, filed on Oct. 28, 2009, and also claims priority fromU.S. Provisional Application No. 61/360,404, filed on Jun. 30, 2010, thecontents of both applications are incorporated herein by reference intheir entireties.

FIELD OF THE INVENTION

The present invention relates to lithographic apparatuses and processes,and more particularly to methods for optimizing illumination sources andmasks for use in lithographic apparatuses and processes.

BACKGROUND OF THE RELATED ART

Lithographic apparatuses can be used, for example, in the manufacture ofintegrated circuits (ICs). In such a case, the mask may contain acircuit pattern corresponding to an individual layer of the IC, and thispattern can be imaged onto a target portion (e.g. comprising one or moredies) on a substrate (silicon wafer) that has been coated with a layerof radiation-sensitive material (resist). In general, a single waferwill contain a whole network of adjacent target portions that aresuccessively irradiated via the projection system, one at a time. In onetype of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionin one go; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus, commonly referred to as a step-and-scanapparatus, each target portion is irradiated by progressively scanningthe mask pattern under the projection beam in a given referencedirection (the “scanning” direction) while synchronously scanning thesubstrate table parallel or anti-parallel to this direction. Since, ingeneral, the projection system will have a magnification factor M(generally <1), the speed V at which the substrate table is scanned willbe a factor M times that at which the mask table is scanned. Moreinformation with regard to lithographic devices as described herein canbe gleaned, for example, from U.S. Pat. No. 6,046,792, incorporatedherein by reference.

In a manufacturing process using a lithographic projection apparatus, amask pattern is imaged onto a substrate that is at least partiallycovered by a layer of radiation-sensitive material (resist). Prior tothis imaging step, the substrate may undergo various procedures, such aspriming, resist coating and a soft bake. After exposure, the substratemay be subjected to other procedures, such as a post-exposure bake(PEB), development, a hard bake and measurement/inspection of the imagedfeatures. This array of procedures is used as a basis to pattern anindividual layer of a device, e.g., an IC. Such a patterned layer maythen undergo various processes such as etching, ion-implantation(doping), metallization, oxidation, chemo-mechanical polishing, etc.,all intended to finish off an individual layer. If several layers arerequired, then the whole procedure, or a variant thereof, will have tobe repeated for each new layer. Eventually, an array of devices will bepresent on the substrate (wafer). These devices are then separated fromone another by a technique such as dicing or sawing, whence theindividual devices can be mounted on a carrier, connected to pins, etc.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection systems,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Twin stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441,incorporated herein by reference.

The photolithographic masks referred to above comprise geometricpatterns corresponding to the circuit components to be integrated onto asilicon wafer. The patterns used to create such masks are generatedutilizing CAD (computer-aided design) programs, this process often beingreferred to as EDA (electronic design automation). Most CAD programsfollow a set of predetermined design rules in order to create functionalmasks. These rules are set by processing and design limitations. Forexample, design rules define the space tolerance between circuit devices(such as gates, capacitors, etc.) or interconnect lines, so as to ensurethat the circuit devices or lines do not interact with one another in anundesirable way. The design rule limitations are typically referred toas “critical dimensions” (CD). A critical dimension of a circuit can bedefined as the smallest width of a line or hole or the smallest spacebetween two lines or two holes. Thus, the CD determines the overall sizeand density of the designed circuit. Of course, one of the goals inintegrated circuit fabrication is to faithfully reproduce the originalcircuit design on the wafer (via the mask).

As noted, microlithography is a central step in the manufacturing ofsemiconductor integrated circuits, where patterns formed onsemiconductor wafer substrates define the functional elements ofsemiconductor devices, such as microprocessors, memory chips etc.Similar lithographic techniques are also used in the formation of flatpanel displays, micro-electro mechanical systems (MEMS) and otherdevices.

As semiconductor manufacturing processes continue to advance, thedimensions of circuit elements have continually been reduced while theamount of functional elements, such as transistors, per device has beensteadily increasing over decades, following a trend commonly referred toas ‘Moore's law’. At the current state of technology, critical layers ofleading-edge devices are manufactured using optical lithographicprojection systems known as scanners that project a mask image onto asubstrate using illumination from a deep-ultraviolet laser light source,creating individual circuit features having dimensions well below 100nm, i.e. less than half the wavelength of the projection light.

This process in which features with dimensions smaller than theclassical resolution limit of an optical projection system are printed,is commonly known as low-k₁ lithography, according to the resolutionformula CD=k₁×λ/NA, where λ is the wavelength of radiation employed(currently in most cases 248 nm or 193 nm), NA is the numerical apertureof the projection optics, CD is the ‘critical dimension’—generally thesmallest feature size printed—and k₁ is an empirical resolution factor.In general, the smaller k₁, the more difficult it becomes to reproduce apattern on the wafer that resembles the shape and dimensions planned bya circuit designer in order to achieve particular electricalfunctionality and performance. To overcome these difficulties,sophisticated fine-tuning steps are applied to the projection system aswell as to the mask design. These include, for example, but not limitedto, optimization of NA and optical coherence settings, customizedillumination schemes, use of phase shifting masks, optical proximitycorrection in the mask layout, or other methods generally defined as‘resolution enhancement techniques’ (RET).

As one important example, optical proximity correction (OPC, sometimesalso referred to as ‘optical and process correction’) addresses the factthat the final size and placement of a printed feature on the wafer willnot simply be a function of the size and placement of the correspondingfeature on the mask. It is noted that the terms ‘mask’ and ‘reticle’ areutilized interchangeably herein. For the small feature sizes and highfeature densities present on typical circuit designs, the position of aparticular edge of a given feature will be influenced to a certainextent by the presence or absence of other adjacent features. Theseproximity effects arise from minute amounts of light coupled from onefeature to another. Similarly, proximity effects may arise fromdiffusion and other chemical effects during post-exposure bake (PEB),resist development, and etching that generally follow lithographicexposure.

In order to ensure that the features are generated on a semiconductorsubstrate in accordance with the requirements of the given targetcircuit design, proximity effects need to be predicted utilizingsophisticated numerical models, and corrections or pre-distortions needto be applied to the design of the mask before successful manufacturingof high-end devices becomes possible. The article “Full-Chip LithographySimulation and Design Analysis—How OPC Is Changing IC Design”, C.Spence, Proc. SPIE, Vol. 5751, pp 1-14 (2005) provides an overview ofcurrent ‘model-based’ optical proximity correction processes. In atypical high-end design almost every feature edge requires somemodification in order to achieve printed patterns that come sufficientlyclose to the target design. These modifications may include shifting orbiasing of edge positions or line widths as well as application of‘assist’ features that are not intended to print themselves, but willaffect the properties of an associated primary feature.

The application of model-based OPC to a target design requires goodprocess models and considerable computational resources, given the manymillions of features typically present in a chip design. However,applying OPC is generally not an ‘exact science’, but an empirical,iterative process that does not always resolve all possible weaknesseson a layout. Therefore, post-OPC designs, i.e. mask layouts afterapplication of all pattern modifications by OPC and any other resolutionenhancement techniques (RET's), need to be verified by designinspection, i.e. intensive full-chip simulation using calibratednumerical process models, in order to minimize the possibility of designflaws being built into the manufacturing of a mask set. This is drivenby the enormous cost of making high-end mask sets, which run in themulti-million dollar range, as well as by the impact on turn-around timeby reworking or repairing actual masks once they have been manufactured.

Both OPC and full-chip RET verification may be based on numericalmodeling systems and methods as described, for example in, U.S. Pat. No.7,003,758 and an article titled “Optimized Hardware and Software ForFast, Full Chip Simulation”, by Y. Cao et al., Proc. SPIE, Vol. 5754,405 (2005).

In addition to performing the foregoing mask adjustments (e.g., OPC) inan effort to optimize the imaging results, the illumination schemeutilized in the imaging process can be also optimized, either jointlywith mask optimization or separately, in an effort to improve theoverall lithography fidelity. Since the 1990s, many off-axis lightsources, such as annular, quadrupole, and dipole, have been introduced,and have provided more freedom for OPC design, thereby improving theimaging results. As is known, off-axis illumination is a proven way toresolve fine structures (i.e., target features) contained in the mask.However, when compared to a traditional illuminator, an off-axisilluminator usually provides less light intensity for the aerial image(AI). Thus, it becomes necessary to attempt to optimize the illuminatorto achieve the optimal balance between finer resolution and reducedlight intensity.

Numerous prior art illumination optimization approaches are known. Forexample, in an article by Rosenbluth et al., titled “Optimum Mask andSource Patterns to Print A Given Shape”, Journal of Microlithography,Microfabrication, Microsystems 1(1), pp. 13-20, (2002), the source ispartitioned into several regions, each of which corresponds to a certainregion of the pupil spectrum. Then, the source distribution is assumedto be uniform in each source region and the brightness of each region isoptimized for process window. However, such an assumption that thesource distribution is uniform in each source region is not alwaysvalid, and as a result the effectiveness of this approach suffers. Inanother example set forth in an article by Granik, titled “SourceOptimization for Image Fidelity and Throughput”, Journal ofMicrolithography, Microfabrication, Microsystems 3(4), pp. 509-522,(2004), several existing source optimization approaches are overviewedand a method based on illuminator pixels is proposed that converts thesource optimization problem into a series of non-negative least squareoptimizations. Though these methods have demonstrated some successes,they typically require multiple complicated iterations to converge. Inaddition, it may be difficult to determine the appropriate/optimalvalues for some extra parameters, such as γ in Granik's method, whichdictates the trade-off between optimizing the source for wafer imagefidelity and the smoothness requirement of the source.

For low k1 photolithography, optimization of both source and mask (i.e.source and mask optimization or SMO) is needed to ensure a viableprocess window for printing critical patterns. Existing algorithms (e.g.Socha et. al. Proc. SPIE vol. 5853, 2005, p. 180) generally discretizeillumination into independent source points and mask into diffractionorders in the spatial frequency domain, and separately formulate a costfunction based on process window metrics such as exposure latitude whichcan be predicted by optical imaging models from source point intensitiesand mask diffraction orders. Then standard optimization techniques areused to minimize the objective function.

Such conventional SMO techniques are computationally expensive,especially for complicated designs. Accordingly, it is generally onlypractical to perform source optimization for simple repeating designssuch as memory designs (Flash, DRAM and SRAM). Meanwhile, the full chipincludes other more complicated designs such as logic and gates. So,since the SMO source optimization is only based on limited small areasof certain designs, it is difficult to guarantee that the source willwork well for the designs that are not included in the SMO process.Accordingly, a need remains for a technique that can optimize a sourcefor multiple clips of designs representing all the complicated designlayouts in the full chip within a practical amount of run time.

SUMMARY OF THE INVENTION

The present invention relates to lithographic apparatuses and processes,and more particularly to tools for optimizing illumination sources andmasks for use in lithographic apparatuses and processes. According tocertain aspects, the present invention enables full chip patterncoverage while lowering the computation cost by intelligently selectinga small set of critical design patterns from the full set of clips to beused in source and mask optimization. Optimization is performed only onthese selected patterns to obtain an optimized source. The optimizedsource is then used to optimize the mask (e.g. using OPC andmanufacturability verification) for the full chip, and the processwindow performance results are compared. If the results are comparableto conventional full-chip SMO, the process ends, otherwise variousmethods are provided for iteratively converging on the successfulresult.

In furtherance of these and other aspects, a method for optimizing alithographic process for imaging a portion of a design onto a waferincludes identifying a full set of clips from the design, selecting asubset of clips from the full set of clips, optimizing an illuminationsource for the lithographic process for imaging the selected subset ofclips, and using the optimized illumination source for optimizing thefull set of clips for being imaged in the lithographic process.

In additional furtherance of the above and other aspects, the selectingstep of the method includes calculating diffraction order distributionsfor each of the full set of clips, grouping the full set of clips into aplurality of groups based on the calculated diffraction orderdistributions, and selecting one or more representative clips from eachof the groups as the subset.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 is an exemplary block diagram illustrating a typical lithographicprojection system.

FIG. 2 is an exemplary block diagram illustrating the functional modulesof a lithographic simulation model.

FIG. 3 is a flowchart illustrating an example SMO process according toembodiments of the invention.

FIG. 4 is a flowchart illustrating an example pattern selection methodthat can be included in one embodiment of an SMO process according tothe invention.

FIG. 5 is a flowchart illustrating an example pattern selection methodthat can be included in another embodiment of an SMO process accordingto the invention.

FIG. 6 is a flowchart illustrating an example pattern selection methodthat can be included in another embodiment of an SMO process accordingto the invention.

FIG. 7 is a flowchart illustrating an example pattern selection methodthat can be included in another embodiment of an SMO process accordingto the invention.

FIG. 8 is a flowchart illustrating an example pattern selection methodthat can be included in another embodiment of an SMO process accordingto the invention.

FIG. 9 illustrates example diffraction order distribution of clipsselected according to the method in FIG. 8.

FIG. 10 is a graph comparing the process window performance for thevarious pattern selection methods according to the invention.

FIG. 11 is a chart comparing processing run time performance for thevarious pattern selection methods according to the invention.

FIG. 12 is a block diagram that illustrates a computer system which canassist in the implementation of the simulation method of the presentinvention.

FIG. 13 schematically depicts a lithographic projection apparatussuitable for use with the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference tothe drawings, which are provided as illustrative examples of theinvention so as to enable those skilled in the art to practice theinvention. Notably, the figures and examples below are not meant tolimit the scope of the present invention to a single embodiment, butother embodiments are possible by way of interchange of some or all ofthe described or illustrated elements. Moreover, where certain elementsof the present invention can be partially or fully implemented usingknown components, only those portions of such known components that arenecessary for an understanding of the present invention will bedescribed, and detailed descriptions of other portions of such knowncomponents will be omitted so as not to obscure the invention.Embodiments described as being implemented in software should not belimited thereto, but can include embodiments implemented in hardware, orcombinations of software and hardware, and vice-versa, as will beapparent to those skilled in the art, unless otherwise specified herein.In the present specification, an embodiment showing a singular componentshould not be considered limiting; rather, the invention is intended toencompass other embodiments including a plurality of the same component,and vice-versa, unless explicitly stated otherwise herein. Moreover,applicants do not intend for any term in the specification or claims tobe ascribed an uncommon or special meaning unless explicitly set forthas such. Further, the present invention encompasses present and futureknown equivalents to the known components referred to herein by way ofillustration.

Although specific reference may be made in this text to the use of theinvention in the manufacture of ICs, it should be explicitly understoodthat the invention has many other possible applications. For example, itmay be employed in the manufacture of integrated optical systems,guidance and detection patterns for magnetic domain memories,liquid-crystal display panels, thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “reticle”, “wafer” or “die” in thistext should be considered as being replaced by the more general terms“mask”, “substrate” and “target portion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange 5-20 nm).

The term mask as employed in this text may be broadly interpreted asreferring to generic patterning means that can be used to endow anincoming radiation beam with a patterned cross-section, corresponding toa pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Besides theclassic mask (transmissive or reflective; binary, phase-shifting,hybrid, etc.), examples of other such patterning means include:

-   -   a programmable mirror array. An example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the said undiffracted light        can be filtered out of the reflected beam, leaving only the        diffracted light behind; in this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-addressable surface. The required matrix addressing can        be performed using suitable electronic means. More information        on such mirror arrays can be gleaned, for example, from U.S.        Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein        by reference.    -   a programmable LCD array. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference.

Prior to discussing the present invention, a brief discussion regardingthe overall simulation and imaging process is provided. FIG. 1illustrates an exemplary lithographic projection system 10. The majorcomponents are a light source 12, which may be a deep-ultravioletexcimer laser source, illumination optics which define the partialcoherence (denoted as sigma) and which may include specific sourceshaping optics 14, 16 a and 16 b; a mask or reticle 18; and projectionoptics 16 c that produce an image of the reticle pattern onto the waferplane 22. An adjustable filter or aperture 20 at the pupil plane mayrestrict the range of beam angles that impinge on the wafer plane 22,where the largest possible angle defines the numerical aperture of theprojection optics NA=sin(Θ_(max)).

In a lithography simulation system, these major system components can bedescribed by separate functional modules, for example, as illustrated inFIG. 2. Referring to FIG. 2, the functional modules include the designlayout module 26, which defines the target design; the mask layoutmodule 28, which defines the mask to be utilized in the imaging process;the mask model module 30, which defines the model of the mask layout tobe utilized during the simulation process; the optical model module 32,which defines the performance of the optical components of lithographysystem; and the resist model module 34, which defines the performance ofthe resist being utilized in the given process. As is known, the resultof the simulation process produces, for example, predicted contours andCDs in the result module 36.

More specifically, it is noted that the properties of the illuminationand projection optics are captured in the optical model 32 thatincludes, but not limited to, NA-sigma (a) settings as well as anyparticular illumination source shape (e.g. off-axis light sources suchas annular, quadrupole, and dipole, etc.). The optical properties of thephoto-resist layer coated on a substrate—i.e. refractive index, filmthickness, propagation and polarization effects—may also be captured aspart of the optical model 32. The mask model 30 captures the designfeatures of the reticle and may also include a representation ofdetailed physical properties of the mask, as described, for example, inU.S. Pat. No. 7,587,704. Finally, the resist model 34 describes theeffects of chemical processes which occur during resist exposure, PEBand development, in order to predict, for example, contours of resistfeatures formed on the substrate wafer. The objective of the simulationis to accurately predict, for example, edge placements and CDs, whichcan then be compared against the target design. The target design, isgenerally defined as the pre-OPC mask layout, and will be provided in astandardized digital file format such as GDSII or OASIS.

In a typical high-end design almost every feature edge requires somemodification in order to achieve printed patterns that come sufficientlyclose to the target design. These modifications may include shifting orbiasing of edge positions or line widths as well as application of‘assist’ features that are not intended to print themselves, but willaffect the properties of an associated primary feature. Furthermore,optimization techniques applied to the source of illumination may havedifferent effects on different edges and features. Optimization ofillumination sources can include the use of pupils to restrict sourceillumination to a selected pattern of light. The present inventionprovides optimization methods that can be applied to both source andmask configurations.

In general, a method of performing source and mask optimization (SMO)according to embodiments of the invention enables full chip patterncoverage while lowering the computation cost by intelligently selectinga small set of critical design patterns from the full set of clips to beused in SMO. SMO is performed only on these selected patterns to obtainan optimized source. The optimized source is then used to optimize themask (e.g. using OPC and LMC) for the full chip, and the results arecompared. If the results are comparable to conventional full-chip SMO,the process ends, otherwise various methods are provided for iterativelyconverging on the successful result.

One example SMO method according to embodiments of the invention will beexplained in connection with the flowchart in FIG. 3.

A target design 300 (typically comprising a layout in a standard digitalformat such as OASIS, GDSII, etc.) for which a lithographic process isto be optimized includes memory, test patterns and logic. From thisdesign, a full set of clips 302 is extracted, which represents all thecomplicated patterns in the design 300 (typically about 50 to 1000clips). As will be appreciated by those skilled in the art, these clipsrepresent small portions (i.e. circuits, cells or patterns) of thedesign for which particular attention and/or verification is needed.

As generally shown in 304, a small subset of clips 306 (e.g. 15 to 50clips) is selected from the full set 302. As will be explained in moredetail below, the selection of clips is preferably performed such thatthe process window of the selected patterns as closely as possiblematches the process window for the full set of critical patterns. Theeffectiveness of the selection is also measured by the total turn runtime (pattern selection and SMO) reduction.

In 308, SMO is performed with the selected patterns (15 to 50 patterns)306. More particularly, an illumination source is optimized for theselected patterns 306. This optimization can be performed using any of awide variety of known methods, for example those described in U.S.Patent Pub. No. 2004/0265707, the contents of which are incorporatedherein by reference.

In 310, manufacturability verification of the selected patterns 306 isperformed with the source obtained in 308. More particularly,verification includes performing an aerial image simulation of theselected patterns 306 and the optimized source and verifying that thepatterns will print across a sufficiently wide process window. Thisverification can be performed using any of a wide variety of knownmethods, for example those described in U.S. Pat. No. 7,342,646, thecontents of which are incorporated herein by reference.

If the verification in 310 is satisfactory, as determined in 312, thenprocessing advances to full chip optimization in 314. Otherwise,processing returns to 308, where SMO is performed again but with adifferent source or set of patterns. For example, the processperformance as estimated by the verification tool can be comparedagainst thresholds for certain process window parameters such asexposure latitude and depth of focus. These thresholds can bepredetermined or set by a user.

In 316, after the selected patterns meet lithography performance spec asdetermined in 312, the optimized source 314 will be used foroptimization of the full set of clips.

In 318, model-based sub-resolution assist feature placement (MB-SRAF)and optical proximity correction (OPC) for all the patterns in the fullset of clips 316 is performed. This process can be performed using anyof a wide variety of known methods, for example those described in U.S.Pat. Nos. 5,663,893, 5,821,014, 6,541,167 and 6,670,081.

In 320, using processes similar to step 310, full pattern simulationbased manufacturability verification is performed with the optimizedsource 314 and the full set of clips 316 as corrected in 318.

In 322, the performance (e.g. process window parameters such as exposurelatitude and depth of focus) of the full set of clips 316 is comparedagainst the subset of clips 306. In one example embodiment, the patternselection is considered complete and/or the source is fully qualifiedfor the full chip when the similar (<10%) lithography performances areobtained for both selected patterns (15 to 20) 306 and all criticalpatterns (50 to 1000) 316.

Otherwise, in 324, hotspots are extracted, and in 326 these hotspots areadded to the subset 306, and the process starts over. For example,hotspots (i.e. features among the full set of clips 316 that limitprocess window performance) identified during verification 320 are usedfor further source tuning or to re-run SMO. The source is consideredfully converged when the process window of the full set of clips 316 arethe same between the last run and the run before the last run of 322.

Multiple pattern selection methods have been developed for use in 304,and certain non-limiting examples are detailed below.

In a first embodiment, a source is optimized for SRAM patterns in thetarget design, then hotspots among the full set of clips are identifiedand selected as the subset of patterns for SMO.

For example, as shown in FIG. 4, the pattern selection according to thisembodiment begins in S402 by selecting SRAM patterns from the targetdesign 300, for example two SRAM patterns.

In step S404, source optimization such as that performed in 308 isperformed using these two patterns to obtain an optimized source for theSRAM patterns.

In step S406, OPC is performed on the full set of clips 302 using theoptimized source from S404. The OPC process performed in this step canbe similar to that described above in connection 318 of FIG. 3.

In step S408, manufacturability verification is performed for the fullset of clips 302 that have been adjusted in S406. This verification canbe performed similarly to that described above in connection with 320 inFIG. 3.

From the manufacturability verification results, the clips having theworst performance are selected in S410. For example, S410 includesidentifying from the manufacturability verification results the five tofifteen clips that have the most limiting effect on the process windowfor the SRAM-optimized source.

The SRAM patterns and hotspots are then used as the subset 306 in theexample full-chip SMO flow of FIG. 3.

In a next embodiment, with an original source and model, hotspots areidentified from the full set of clips, and these are selected as thesubset of patterns for SMO.

For example, as shown in FIG. 5, the pattern selection according to thisembodiment begins in S502 by identifying the original source and modelfor the lithographic process. For example, an annular illuminationsource is used as the initial source. The model can be any model of thelithographic process used in computational lithography and aerial imagesimulation, and can include Transmission Cross Coefficients (TCCs) asdescribed, for example in U.S. Pat. No. 7,342,646.

In step S504, manufacturability verification is performed using thesource and model and the full set of clips 302. The verificationprocessing can be similar to that described above in connection with 310in FIG. 3.

In step S506, a severity score is calculated using the verificationresults for each of the full set of clips 302 to identify hotspots. Inone non-limiting example, the severity score is calculated as:Score=Normalized (+EPE)+Normalized (−EPE)+2*Normalized MEEFwhere EPE is edge placement error and MEEF is mask error enhancementfactor.

In step S508, the clips having the highest score are identified ashotspots. For example, S508 includes identifying the five to fifteenclips that have the highest severity score as calculated above.

These clips are then used as the subset 306 in the example full-chip SMOflow of FIG. 3. In embodiments, two SRAM patterns from target design 300are also included in the subset 306.

In a next embodiment, an analysis is performed on the full set of clips302, and those clips giving the best feature and pitch coverage areselected as the subset of patterns for SMO.

For example, as shown in FIG. 6, the pattern selection according to thisembodiment begins in S602 by grouping the clips according to featuretype. For example, the clips can be grouped by the type of circuitpattern (e.g. gates or logic) or by orientation or complexity, etc.

In step S604, the clips in each group are further sorted by pitch.

In step S606, each of the clips is sampled in the small pitch zone todetermine the coverage that will be provided for both type and pitch.

In step S608, the clips having the minimum pitch and highest celldensity are selected from among those giving the desired coverage in5606. For example, S608 includes identifying the five to fifteen clipsthat have the best design coverage and pitches from minimum to 1.5 timesthe minimum pitch.

These clips are then used as the subset 306 in the example full-chip SMOflow of FIG. 3. In embodiments, two SRAM patterns from target design 300are also included in the subset 306.

In a next embodiment, an analysis is performed on the full set of clips,and those clips having the highest sensitivity to certain processparameters according to an original model of the process are selected asthe subset of patterns for SMO.

For example, as shown in FIG. 7, the pattern selection according to thisembodiment begins in 5702 by identifying the original model for thelithography process. Similar to S502, the model can be any model of thelithographic process used in computational lithography and aerial imagesimulation, and can include Transmission Cross Coefficients (TCCs) asdescribed, for example in U.S. Pat. No. 7,342,646.

In step S704, cut-lines are placed in patterns located at the center ofeach of the full set of clips 302.

In step S706, process parameter sensitivities are calculated for each ofthe clips using the original model. For example, the process parameterscan be dose and focus, and the sensitivities can be calculated byrunning aerial image simulation using the lithographic processsimulation model identified in S702. The behavior of the clips at thecut lines during various process conditions are then analyzed todetermine their sensitivities.

In step S708, the clips having the highest sensitivity to processparameter variations are selected. For example, 5708 includesidentifying the five to fifteen clips that have the highest sensitivityto changes in dose and focus.

These clips are then used as the subset 306 in the example full-chip SMOflow of FIG. 3. In embodiments, two SRAM patterns from target design 300are also included in the subset 306.

In a next embodiment, an analysis is performed on the full set of clips,and those clips providing the best diffraction order distribution areselected as the subset of patterns for SMO. Diffraction orders ofpatterns are known to those skilled in the art, and can be determinedfor example, as described in U.S. Patent Pub. No. 2004/0265707.

For example, as shown in FIG. 8, the pattern selection according to thisembodiment begins in S802 by calculating the diffraction order behaviorfor each of the full set of clips 302. Numerous possible methods can beused to calculate the diffraction order behavior, for example, U.S.Patent Pub. No. 2004/0265707.

In step S804, the calculated diffraction orders of the full set of clipsare compared and in step S806, the clips are grouped according to theirdiffraction order distribution. For example, a geometrical correlationbetween each of the clips can be calculated, and sorting methods can beperformed to group the most similar clips together.

In step S808, one clip from each of the groups are selected. Forexample, S806 includes forming five to fifteen groups of clips, and oneclip is randomly selected from each group. FIGS. 9A-N and 9P illustratesan example diffraction order distributions 902A-N and 902P for fifteenindividual clips that have been calculated from a set of full clips.

These clips are then used as the subset 306 in the example full-chip SMOflow of FIG. 3. In embodiments, two SRAM patterns from target design 300are also included in the subset 306.

Some advantages of the diffraction order based pattern selection methoddescribed in connection with FIG. 8 versus the other methods are that nostarting condition is required (e.g. starting illumination source), noresist model is required, and no models are required. It only requiresthe target pattern, so it is process-independent.

FIG. 10 is a graph comparing the process window performance for thevarious pattern selection methods described above, versus a conventionalfull-chip SMO method. As can be seen, all methods improve upon theoriginal process window, with the diffraction order method giving theclosest performance to the full-chip SMO.

FIG. 11 is a chart comparing processing run time performance for thevarious pattern selection methods described above, versus a conventionalfull-chip SMO method. As can be seen, all methods improve upon theconventional run time, with the diffraction order method giving the mostimprovement.

FIG. 12 is a block diagram that illustrates a computer system 100 whichcan assist in implementing the optimization methods and flows disclosedherein. Computer system 100 includes a bus 102 or other communicationmechanism for communicating information, and a processor(s) 104 (and105) coupled with bus 102 for processing information. Computer system100 also includes a main memory 106, such as a random access memory(RAM) or other dynamic storage device, coupled to bus 102 for storinginformation and instructions to be executed by processor 104. Mainmemory 106 also may be used for storing temporary variables or otherintermediate information during execution of instructions to be executedby processor 104. Computer system 100 further includes a read onlymemory (ROM) 108 or other static storage device coupled to bus 102 forstoring static information and instructions for processor 104. A storagedevice 110, such as a magnetic disk or optical disk, is provided andcoupled to bus 102 for storing information and instructions.

Computer system 100 may be coupled via bus 102 to a display 112, such asa cathode ray tube (CRT) or flat panel or touch panel display fordisplaying information to a computer user. An input device 114,including alphanumeric and other keys, is coupled to bus 102 forcommunicating information and command selections to processor 104.Another type of user input device is cursor control 116, such as amouse, a trackball, or cursor direction keys for communicating directioninformation and command selections to processor 104 and for controllingcursor movement on display 112. This input device typically has twodegrees of freedom in two axes, a first axis (e.g., x) and a second axis(e.g., y), that allows the device to specify positions in a plane. Atouch panel (screen) display may also be used as an input device.

According to one embodiment of the invention, portions of theoptimization process may be performed by computer system 100 in responseto processor 104 executing one or more sequences of one or moreinstructions contained in main memory 106. Such instructions may be readinto main memory 106 from another computer-readable medium, such asstorage device 110. Execution of the sequences of instructions containedin main memory 106 causes processor 104 to perform the process stepsdescribed herein. One or more processors in a multi-processingarrangement may also be employed to execute the sequences ofinstructions contained in main memory 106. In alternative embodiments,hard-wired circuitry may be used in place of or in combination withsoftware instructions to implement the invention. Thus, embodiments ofthe invention are not limited to any specific combination of hardwarecircuitry and software.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to processor 104 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media and volatile media. Non-volatile media include,for example, optical or magnetic disks, such as storage device 110.Volatile media include dynamic memory, such as main memory 106. Commonforms of computer-readable media include, for example, a floppy disk, aflexible disk, hard disk, magnetic tape, any other magnetic medium, aCD-ROM, DVD, any other optical medium, a RAM, a PROM, and EPROM, aFLASH-EPROM, any other memory chip or cartridge, or any other mediumfrom which a computer can read.

Various forms of computer readable media may be involved in carrying oneor more sequences of one or more instructions to processor 104 forexecution. For example, the instructions may initially be borne on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 100 canreceive the data on the telephone line and use an infrared transmitterto convert the data to an infrared signal. An infrared detector coupledto bus 102 can receive the data carried in the infrared signal and placethe data on bus 102. Bus 102 carries the data to main memory 106, fromwhich processor 104 retrieves and executes the instructions. Theinstructions received by main memory 106 may optionally be stored onstorage device 110 either before or after execution by processor 104.

Computer system 100 also preferably includes a communication interface118 coupled to bus 102. Communication interface 118 provides a two-waydata communication coupling to a network link 120 that is connected to alocal network 122. For example, communication interface 118 may be anintegrated services digital network (ISDN) card or a modem to provide adata communication connection to a corresponding type of telephone line.As another example, communication interface 118 may be a local areanetwork (LAN) card to provide a data communication connection to acompatible LAN. Wireless links may also be implemented. In any suchimplementation, communication interface 118 sends and receiveselectrical, electromagnetic or optical signals that carry digital datastreams representing various types of information.

Network link 120 typically provides data communication through one ormore networks to other data devices. For example, network link 120 mayprovide a connection through local network 122 to a host computer 124 orto data equipment operated by an Internet Service Provider (ISP) 126.ISP 126 in turn provides data communication services through theworldwide packet data communication network, now commonly referred to asthe “Internet” 128. Local network 122 and Internet 128 both useelectrical, electromagnetic or optical signals that carry digital datastreams. The signals through the various networks and the signals onnetwork link 120 and through communication interface 118, which carrythe digital data to and from computer system 100, are exemplary forms ofcarrier waves transporting the information.

Computer system 100 can send messages and receive data, includingprogram code, through the network(s), network link 120, andcommunication interface 118. In the Internet example, a server 130 mighttransmit a requested code for an application program through Internet128, ISP 126, local network 122 and communication interface 118. Inaccordance with the invention, one such downloaded application providesfor the illumination optimization of the embodiment, for example. Thereceived code may be executed by processor 104 as it is received, and/orstored in storage device 110, or other non-volatile storage for laterexecution. In this manner, computer system 100 may obtain applicationcode in the form of a carrier wave.

FIG. 13 schematically depicts an exemplary lithographic projectionapparatus whose illumination source could be optimized utilizing theprocesses of the present invention. The apparatus comprises:

a radiation system IL, for supplying a projection beam B of radiation.In this particular case, the radiation system also comprises a radiationsource LA;

a first object table (mask table) MT provided with a mask holder forholding a mask MA (e.g., a reticle), and connected to first positioningmeans for accurately positioning the mask with respect to item PS;

a second object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g., a resist-coated silicon wafer),and connected to second positioning means for accurately positioning thesubstrate with respect to item PS;

a projection system (“lens”) PS (e.g., a refractive, catoptric orcatadioptric optical system) for imaging an irradiated portion of themask MA onto a target portion C (e.g., comprising one or more dies) ofthe substrate W.

As depicted herein, the apparatus is of a transmissive type (i.e., has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (with a reflective mask). Alternatively, the apparatusmay employ another kind of patterning means as an alternative to the useof a mask; examples include a programmable mirror array or LCD matrix.

The source SO (e.g., a mercury lamp or excimer laser) produces a beam ofradiation. This beam is fed into an illumination system (illuminator)IL, either directly or after having traversed conditioning means, suchas a beam expander BD, for example. The illuminator IL may compriseadjusting means AD for setting the outer and/or inner radial extent(commonly referred to as σ-outer and σ-inner, respectively) of theintensity distribution in the beam. In addition, it will generallycomprise various other components, such as an integrator IN and acondenser CO. In this way, the beam B impinging on the mask MA has adesired uniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 13 that the source SO may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source SO is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam that it produces being led into the apparatus (e.g., withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source SO is an excimer laser (e.g., based on KrF, ArFor F₂ lasing). The current invention encompasses at least both of thesescenarios.

The beam B subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam B passes through thelens PS, which focuses the beam B onto a target portion C of thesubstrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the beam B. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamB, e.g., after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 13. However, in the case of a wafer stepper (as opposedto a step-and-scan tool) the mask table MT may just be connected to ashort stroke actuator, or may be fixed. Patterning device MA andsubstrate W may be aligned using alignment marks M1, M2 in thepatterning device, and alignment marks P1, P2 on the wafer, as required.

The depicted tool can be used in two different modes:

-   -   In step mode, the mask table MT is kept essentially stationary,        and an entire mask image is projected in one go (i.e., a single        “flash”) onto a target portion C. The substrate table WT is then        shifted in the x and/or y directions so that a different target        portion C can be irradiated by the beam B;    -   In scan mode, essentially the same scenario applies, except that        a given target portion C is not exposed in a single “flash”.        Instead, the mask table MT is movable in a given direction (the        so-called “scan direction”, e.g., the y direction) with a speed        v, so that the projection beam B is caused to scan over a mask        image; concurrently, the substrate table WT is simultaneously        moved in the same or opposite direction at a speed V=Mv, in        which M is the magnification of the lens PS (typically, M=¼ or        ⅕). In this manner, a relatively large target portion C can be        exposed, without having to compromise on resolution.

The concepts disclosed herein may simulate or mathematically model anygeneric imaging system for imaging sub wavelength features, and may beespecially useful with emerging imaging technologies capable ofproducing wavelengths of an increasingly smaller size. Emergingtechnologies already in use include EUV (extreme ultra violet)lithography that is capable of producing a 193 nm wavelength with theuse of a ArF laser, and even a 157 nm wavelength with the use of aFluorine laser. Moreover, EUV lithography is capable of producingwavelengths within a range of 20-5 nm by using a synchrotron or byhitting a material (either solid or a plasma) with high energy electronsin order to produce photons within this range. Because most materialsare absorptive within this range, illumination may be produced byreflective mirrors with a multi-stack of Molybdenum and Silicon. Themulti-stack mirror has a 40 layer pairs of Molybdenum and Silicon wherethe thickness of each layer is a quarter wavelength. Even smallerwavelengths may be produced with X-ray lithography. Typically, asynchrotron is used to produce an X-ray wavelength. Since most materialis absorptive at x-ray wavelengths, a thin piece of absorbing materialdefines where features would print (positive resist) or not print(negative resist).

While the concepts disclosed herein may be used for imaging on asubstrate such as a silicon wafer, it shall be understood that thedisclosed concepts may be used with any type of lithographic imagingsystems, e.g., those used for imaging on substrates other than siliconwafers.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

The invention may be further described using the following clauses:

1. A computer readable medium having instructions recorded thereon,which when read by a computer, causes the computer to perform a methodfor optimizing a lithographic process for imaging a portion of a designonto a substrate, the method comprising:

selecting a subset of patterns from the portion of the design;

optimizing an illumination source for the lithographic process forimaging the selected subset of patterns; and

using the optimized illumination source for optimizing the portion ofthe design for being imaged in the lithographic process.

2. A computer readable medium according to clause 1, wherein the portionof the design comprises clips and wherein the step of selecting a subsetof patterns comprises:

identifying a full set of clips from the design;

selecting a subset of clips from the full set of clips;

wherein the step of optimizing comprises optimizing an illuminationsource for the lithographic process for imaging the selected subset ofclips; and

wherein the step of using comprises using the optimized illuminationsource for optimizing the full set of clips for being imaged in thelithographic process.

3. A computer readable medium according to clause 1 or 2, wherein theselecting step includes:

calculating diffraction order distributions for the patterns in theportion of the design;

grouping said patterns into a plurality of groups based on thecalculated diffraction order distributions; and

selecting one or more representative patterns from each of the groups asthe subset of patterns.

4. A computer readable medium according to clause 1 or 2, wherein theselecting step includes:

identifying one or more memory patterns in the portion of the design;

pre-optimizing the illumination source for the one or more memorypatterns;

using the pre-optimized illumination source to determine potential hotspots in the portion of the design; and

selecting the subset of patterns based on the determined potential hotspots.

5. A computer readable medium according to clause 1 or 2, wherein theselecting step includes:

identifying an original illumination source for the lithographicprocess;

using the original illumination source to determine potential hot spotsin the portion of the design; and

selecting the subset of patterns based on the determined potential hotspots.

6. A method according to clause 4 or 5, wherein the method furthercomprises a step of:

calculating a severity score for the hot spots; and

selecting the hot spots having a predefined severity score or having apredefined severity score range.

7. A computer readable medium according to clause 1 or 2, wherein theselecting step includes:

grouping patterns in the portion of the design by design type into aplurality of groups;

sorting the patterns in each group by pitch and feature type todetermine an optimal pattern in each group; and

selecting the optimal pattern in each group as the subset of patterns.

8. A computer readable medium according to clause 1 or 2, wherein theselecting step includes:

identifying a simulation model of the lithographic process;

using the model to estimate process parameter sensitivities for patternsin the portion of the design; and

selecting the subset of patterns based on the estimated processparameter sensitivities.

9. A computer readable medium according to any of the clauses 1 to 8,further comprising:

determining whether a lithographic process performance metric for theoptimized subset of patterns is acceptable; and

if the determined metric is not acceptable, adding clips havingpotential hot spots to the subset and repeating the optimization steps.

10. A computer readable medium according to any of the clauses 1 to 9,wherein the step of optimizing the illumination source includessimulating a lithographic process performance using a model of thelithographic process, the illumination source, and the subset ofpatterns to determine whether the performance is acceptable.11. A computer readable medium according to any the clauses 1 to 10,wherein the step of optimizing the portion of the design includesperforming optical proximity correction on certain of the patterns basedon the optimized illumination source.

The invention claimed is:
 1. A method implemented by a computer, themethod being for optimizing a lithographic process for imaging a portionof a design onto a substrate, the method comprising: selecting a subsetof patterns from the portion of the design; optimizing an illuminationsource for the lithographic process for imaging the selected subset ofpatterns, wherein said optimizing the illumination source comprisesdetermining, by the computer, a configuration of the illumination sourcethat is capable of producing a desired image of the selected subset ofpatterns; and using the optimized illumination source for optimizing theportion of the design for being imaged in the lithographic process,wherein said optimizing the portion of the design comprises determining,by the computer, which features of the portion of the design requirealteration for producing a desired image of the portion of the designwhen the optimized illumination source is used.
 2. A method according toclaim 1, wherein the portion of the design comprises a full chip.
 3. Amethod according to claim 1, wherein the portion of the design comprisesclips and wherein the step of selecting a subset of patterns comprises:identifying a full set of clips from the design; selecting a subset ofclips from the full set of clips; wherein the step of optimizingcomprises optimizing an illumination source for the lithographic processfor imaging the selected subset of clips; and wherein the step of usingcomprises using the optimized illumination source for optimizing thefull set of clips for being imaged, in the lithographic process.
 4. Amethod according to claim 1, wherein the selecting step includes:calculating diffraction order distributions for patterns in the portionof the design; grouping said patterns into a plurality of groups basedon the calculated diffraction order distributions; and selecting one ormore representative patterns from each of the groups as the subset ofpatterns.
 5. A method according to claim 1, wherein the selecting stepincludes: identifying one or more memory patterns in the portion of thedesign; pre-optimizing the illumination source for the one or morememory patterns; using the pre-optimized illumination source todetermine potential hot spots in the portion of the design; andselecting the subset of patterns based on the determined potential hotspots.
 6. A method according to claim 1, wherein the selecting stepincludes: identifying an original illumination source for thelithographic process; using the original illumination source todetermine potential hot spots in the portion of the design; andselecting the subset of patterns based on the determined potential hotspots.
 7. A method according to claim 5, wherein the method furthercomprises a step of: calculating a severity score for a hot spot; andselecting the hot spot having a predefined severity score or selectingthe hot spot having a severity score within a predefined severity scorerange.
 8. A method according to claim 6, wherein the method furthercomprises a step of: calculating a severity score for a hot spot; andselecting the hot spot having a predefined severity score or selectingthe hot spot having a severity score within a predefined severity scorerange.
 9. A method according to claim 1, wherein the selecting stepincludes: grouping patterns in the portion of the design by design typeinto a plurality of groups; sorting the patterns in each group by pitchand feature type to determine an optimal pattern in each group; andselecting the optimal pattern in each group as the subset of patterns.10. A method according to claim 1, wherein the selecting step includes:identifying a simulation model of the lithographic process; using themodel to estimate process parameter sensitivities for patterns in theportion of the design; and selecting the subset of patterns based on theestimated process parameter sensitivities.
 11. A method according toclaim 1, further comprising: determining whether a lithographic processperformance metric for the optimized subset of patterns is acceptable;and if the determined metric is not acceptable, adding clips havingpotential hot spots to the subset of patterns and repeating theoptimization steps.
 12. A method according to claim 1, wherein the stepof optimizing the illumination source includes simulating a lithographicprocess performance using a model of the lithographic process, theillumination source, and the subset of patterns to determine whether theperformance is acceptable.
 13. A method according to claim 1, whereinthe step of optimizing the portion of the design includes performingoptical proximity correction on certain of the patterns based on theoptimized illumination source.
 14. A non-transitory computer readablestorage medium having instructions recorded thereon, which when read bya computer, causes the computer to perform a method for optimizing alithographic process for imaging a portion of a design onto a waferaccording to claim
 1. 15. A lithographic apparatus comprising: anillumination system configured to provide a beam of radiation; a supportstructure configured to support a patterning means, the patterning meansserving to impart the radiation beam with a pattern in itscross-section; a substrate table configured to hold a substrate; and aprojection system for projecting the patterned radiation beam onto atarget portion of the substrate; wherein the lithographic apparatusfurther comprises a processor for configuring the illumination system togenerate the optimized illumination source according to the method foroptimizing a lithographic process of claim
 1. 16. A patterning means forimparting a radiation beam from an illumination system of a lithographicapparatus being configured for projecting this imparted beam via aprojection system onto a target portion of a substrate, wherein thepatterning means comprises an optimized portion of a design, wherein theoptimized portion of the design is determined according to the method ofoptimizing a lithographic process of claim
 1. 17. A method foroptimizing a lithographic process for imaging a portion of a design ontoa wafer, the method comprising: identifying a full set of clips from thedesign; selecting a subset of clips from the full set of clips;optimizing an illumination source for the lithographic process forimaging the selected subset of clips, wherein said optimizing theillumination source comprises determining, by the computer, aconfiguration of the illumination source that is capable of producing adesired image of the subset of clips; and using the optimizedillumination source for optimizing the full set of clips for beingimaged in the lithographic process, wherein said optimizing the full setof clips comprises determining, by the computer, which features of thefull set of clips require alteration for producing a desired image ofthe full set of clips when the optimized illumination source is used.18. A method according to claim 17, wherein the selecting step includes:calculating diffraction order distributions for each of the full set ofclips; grouping the full set of clips into a plurality of groups basedon the calculated diffraction order distributions; and selecting one ormore representative clips from each of the groups as the subset.
 19. Amethod according to claim 18, wherein the selecting step includes:identifying one or more memory patterns in the full set of clips;pre-optimizing the illumination source for the one or more memorypatterns; using the pre-optimized illumination source to determinepotential hot spots in the full set of clips; and selecting the subsetbased on the determined potential hot spots.
 20. A method according toclaim 17, wherein the selecting step includes: identifying an originalillumination source for the lithographic process; using the originalillumination source to determine potential hot spots in the full set ofclips; and selecting the subset based on the determined potential hotspots.
 21. A method according to claim 17, wherein the selecting stepincludes: grouping patterns in the full set of clips by design type intoa plurality of groups; sorting the patterns in each group by pitch andfeature type to determine an optimal pattern in each group; andselecting the optimal pattern in each group as the subset.
 22. A methodaccording to claim 17, wherein the selecting step includes: identifyinga simulation model of the lithographic process; using the model toestimate process parameter sensitivities for each of the full set ofclips; and selecting the subset based on the estimated process parametersensitivities.